1. Field of the Invention
This invention relates to the field of acidic amorphous materials suitable for catalytic supports and more specifically to the field of Fischer-Tropsch hydrocracking catalysts comprising acidic amorphous supports.
2. Background of the Invention
Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing, and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require the energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen and/or steam to form synthesis gas, which is a combination of carbon monoxide and hydrogen. In the second transformation, which is known as Fischer-Tropsch synthesis, carbon monoxide is reacted with hydrogen to form organic molecules containing mainly carbon and hydrogen. Those organic molecules containing carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen, which are known as oxygenates, can also be formed during the Fischer-Tropsch synthesis. Hydrocarbons comprising carbons having no ring formation are known as aliphatic hydrocarbons and are particularly desirable as the basis of synthetic diesel fuel.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus has a range of molecular weights. Therefore, the Fischer-Tropsch products produced by conversion of synthesis gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, naphtha, diesel, and jet fuel, as well as heavier middle distillates. Hydrocarbon waxes may be subjected to an additional hydroprocessing step for conversion to a liquid and/or a gaseous hydrocarbon. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reaction zone that may include one or more reactors.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors, either placed in series, in parallel or both. Common reactors include packed bed (also termed fixed bed) reactors and fluidized reactors, such as slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors comprising catalytic particles sometimes called “slurry reactors,” “ebullating bed reactors,” “slurry bed reactors” or “slurry bubble column reactors,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Some of the principal advantages of gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) for the exothermic Fischer-Tropsch synthesis are the very high heat transfer rates, and the ability to remove and add catalyst online. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55), incorporated herein by reference in its entirety, give a history of the development of various Fischer-Tropsch reactors.
An additional processing step for Fischer-Tropsch products is hydrocracking the Fischer-Tropsch wax. Hydrocracking typically includes reacting the wax over hydrocracking catalysts to convert the wax to hydrocarbon gases and/or liquids. The majority of catalysts currently used for hydrocracking as well as hydroisomerization are bi-functional in nature, and typically comprise a hydro-dehydrogenation component and a cracking component (typically an acid component). The hydro-dehydrogenation component may include one or more metals from Groups 8, 9 and 10 of the Periodic Table of elements (according to the new IUPAC notation) such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. The hydro-dehydrogenation component may also typically include a combination of a Group 6 metal such as chromium, tungsten or molybdenum, with a metal from Groups 8, 9, and 10 (typically a non-noble metal such as iron, cobalt, and/or nickel). The cracking component for the hydrocracking catalyst is typically provided by a support with a large surface area (generally between 150 and 800 m2/g) with a superficial acidity such as halogenated (chlorinated or fluorinated) aluminas, phosphorated aluminas, combinations of boron and aluminum oxides, or silica-aluminas, which may be amorphous or crystalline (the morphology being determined typically by means of X-ray diffractometry from powders).
The balance between the two, acid and hydro-dehydrogenation, functions can be a parameter that governs the activity and selectivity of a hydrocracking or hydroisomerization catalyst. A strong acid function combined with a weak hydro-dehydrogenating function typically produces catalysts that are highly active for hydrocracking. Thus, it is desirable to adjust the activity/selectivity balance of the bi-functional catalyst by the judicious choice of each component.
For the hydrocracking of hydrocarbons, the acidity (for instance the acid strength, the acid site distribution, the acidity index, and the like) of the cracking component in the bi-functional catalyst can play a role in the behavior of the bi-functional catalyst. For instance, the acidity of the cracking component typically affects the hydrocarbon conversion, i.e., the percentage of the hydrocarbons passing over the catalyst that get converted to smaller hydrocarbons. Acidity of cracking components in conventional hydrocracking catalysts is typically controlled by the composition of the cracking component, such as varying content in chorine or fluorine atoms in halogenated aluminas; content in phosphorus atoms in phosphorated aluminas; varying content in alumina in a silica-alumina material; or adding alkali elements such as Group 1 elements from the Periodic Table (like sodium, potassium) added to silica-aluminates (such as for example zeolite L). However, the composition of the cracking component is not the sole factor that affects the acidity of the cracking component. The method of preparation that is employed to make the cracking component can also impact its acidity. For example, it is possible to make various amorphous silica-alumina materials of different acidities that contain the same silica-to-alumina molar ratio. A typical method of changing the acidity of a structured aluminosilicate (i.e., zeolite) is to dealuminate the zeolite so as to decrease its alumina content. Drawbacks for these conventional methods include the inability to efficiently control the acidity of the cracking component. Further drawbacks include the need to use modifiers of superficial acidity, which add cost to the manufacturing costs. Moreover, these modifiers may affect the selectivity of the hydrocracking reaction.
Consequently, there is a need for a hydrocracking catalyst having a cracking component with a controlled acid strength, wherein the cracking component acid strength is not altered by the use of superficial acidity modifiers. Additional needs include a method for preparing an improved hydrocracking catalyst, said method allowing more efficient control of the acid function of the hydrocracking catalyst. Further needs include an improved method for hydrocracking of hydrocarbons, particularly of waxy hydrocarbons derived from a Fischer-Tropsch synthesis.